For the extra-corporeal treatment of blood, there are various processes which are used for removing substances needing to be excreted and for extracting fluid. In hemodialysis, the patient's blood is cleansed outside the body in a dialyser. The dialyser has a blood chamber and a dialysis-fluid chamber which are separated by a semi-permeable membrane. During the treatment, to enable the blood to be cleansed effectively, the patient's blood flows through the blood chamber while dialysis fluid flows through the dialysis-fluid chamber.
In hemodialysis (HD), the movement (diffusion) of the substances of low molecular weight through the semi-permeable membrane of the dialyser is governed essentially by the differences in concentration between the blood and the dialysis fluid. In hemofiltration (HF), fluid is extracted from the blood, through the membrane (ultrafiltration), by means of a pressure gradient that is applied to the semi-permeable membrane (a trans-membrane pressure). Hemodiafiltration is a process for the extra-corporeal treatment of blood in which both hemodialysis and hemofiltration are carried out.
The exchange of matter that takes place during the extra-corporeal blood treatment takes place in the dialyser. The form of dialyser most frequently used is the capillary dialyser which comprises a housing in which a large number of hollow fibers are arranged in parallel. The blood flows within the hollow fibers whereas the dialysis fluid flows along the outside of the bundle of hollow fibers. To increase the effectiveness of the transfer of matter, the blood and the dialysis fluid flow in counter-current. An extra-corporeal blood treatment apparatus for hemodialysis, hemofiltration and hemodiafiltration will be referred to in what follows as a dialysis apparatus.
The known pieces of dialysis apparatus have peristaltic pumps by which the blood is pumped through the dialyser in the extra-corporeal blood circuit.
Known from DE 39 23 692 A1 is a medical apparatus for the exchange of matter and heat between a liquid and a treatment medium, in which the pump pumping the treatment medium and the blood treatment unit are arranged one behind the other in an elongated housing. The tubular housing comprises a pump chamber having a pump impeller and a space in which is situated a bundle of hollow fibers whose hollow fibers extend in the longitudinal direction of the tubular housing. As it rotates, the pump impeller pumps the treatment medium through the hollow fibers of the bundle of hollow fibers while a liquid flows round the bundle of hollow fibers.
In the field of blood oxygenation, impeller pumps are used to pump the blood. In oxygenating blood, blood circulating in an extra-corporeal blood circuit is brought into contact with oxygen through a selective membrane. Bundles of hollow fibers are also used as selective membranes. However, in contrast to dialysis, what flows through the hollow fibers is not blood but the treatment medium (oxygen), while the blood flows along the outside of the hollow fibers.
There are various known types of impeller pumps. The particularly distinctive feature of impeller pumps is that they have an impeller which is surrounded by an annular or tubular housing.
WO 93/05828 and EP 1 930 034 A1 describe impeller pumps for blood oxygenation. The two impeller pumps are characterised in that the blood is pumped along the outside of the bundle of hollow fibers by the impeller of the impeller pump while the oxygen flows through the hollow fibers.